The long term objective of this study is to determine the chemical mechanisms by which ionizing radiation causes alterations in the primary structure of DNA. The specific aim, which provides a means of working toward this objective, is to formulate a set of rules that will predict, quantitatively and qualitatively, the free radical events in DNA model compounds. Two aspects of this problem will be emphasized. One is to identify free radical reactions centered on the sugar-phosphate moiety that result in breakage of the DNA polymer backbone. The other is to determine how the chemical environment influences radiation damage of the bases. The model systems under study reflect this emphasis. Using electron spin resonance and electron nuclear double resonance, single crystals are studied between 6K and 300K. Current investigations are centered around the study of the deoxynucleotide 5'-deoxycytidytic acid, the base complexes 5'-fluorouracil:1-methylcytosine and 5-bromouracil:adenosine, and isolated bases of 1-methylcytosine and 1-methyluracil. Achievement of the specific aims, and eventually the long term objective, provides an important part of the ground work necessary for assessing the biological consequences of exposure to ionizing radiation. This very basis approach may be one of the few routes by which to answer some important questions not solvable by experimentation on animals. For example, in trying to predict the effects of long term exposure to low levels of ionizing radiation, experiments using animals become impractical. On the other hand, a good beginning in solving this problem is obtained once we know just what type and frequency of damage is produced in the genetic material.